X-ray tube current stabilization



July 21, 1970 w. E. SPLAIN X-RAY TUBE CURRENT STABILIZATION 2 Sheets-Sheet 2 Filed April 15, 1968 m: r E

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INVENTOR.

WALTER SPLAIN BY m i ATTORNEYS.

United States Patent 3,521,067 X-RAY TUBE CURRENT STABILIZATION Walter E. Splain, Solon, Ohio, assignor to Picker Corporation, White Plains, N.Y., a corporation of New York Filed Apr. 15, 1968, Ser. No. 721,484 Int. Cl. Hg 1/34 US. Cl. 250-103 30 Claims ABSTRACT OF THE DISCLOSURE In an X-ray tube current stabilization system, fluoroscopic stabilization and pre-exposure and actual exposure radiographic stabilization are provided by X-ray tube filament current control. In the pre-exposure mode, filament current is controlled in accordance with selected tube high voltage and current values utilizing actual filament current feedback. In the exposure mode, filament current is controlled to provide a selected current through the X-ray tube utilizing actual tube current feedback. During fluoroscopy, filament current is controlled in accordance with a predetermined value utilizing actual filament current feedback.

RELATED PATENTS AND APPLICATIONS (1) US. Pat. No. 3,325,645, issued June 13, 1967 to W. E. Splain for X-Ray Tube System With Voltage and Current Control Means;

(2) US. patent application Ser. No. 651,835, filed July 7, 1967 by W. E. Splain for Grid-Controlled X-Ray Tube Control System;

(3) US. patent application Ser. No. 708,963, filed Feb. 28, 1968 by W. E. Splain for Current Measuring Circuit With Capacity Current Compensation.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to X-ray apparatus, and more particularly to a system for stabilizing X-ray tube current by controlling its filament heating current and thus the electron emission rate of the filament.

Discussion of the prior art It is known in the art to provide control means for controlling both the high voltage provided to an X-ray tube and for controlling the current through the tube. Among such systems is that disclosed in the referenced Pat. No. 3,325,645. The system described in that patent is a rather sophisticated one and includes, among numerous other features, stabilization of the X-ray tube current by a control circuit which adjusts the X-ray tube filament current in response to changes in the current in the X-ray tube circuit. The X-ray tube high voltage is stabilized by subtracting a correcting voltage from the rectified voltage applied to the X-ray tube. Other systems for controlling X-ray tube current by means of filament current control have also been proposed.

All of the X-ray tube current stabilization systems in general use today have suffered from one deficiency, however. That deficiency is that they have no provision for preheating the X-ray tube filament, so that it emits an electron beam of the proper current density immediately upon application of high voltage to the tube. This is required where exposures are to be made that are short compared to the thermal time constant of the X-ray tube filament structure. The problem and its solution are further complicated by the so-called space charge effect. It is that effect, which is inherent in presently known X-ray tubes, that necessitates increasing the X-ray tube filament temperature (and hence the filament current) to maintain a ice constant tube current as the high voltage across the tube is reduced.

Accordingly, it is a general object of the present invention to provide a stabilization system for an X-ray tube, wherein filament current is provided to the X-ray tube before an exposure is made in accordance with preselected high voltage and tube current values.

It is another object of the invention to provide a system embodying the foregoing feature, in which during an actual exposure the filament current is controlled to maintain a preselected current through the X-ray tube.

It is a further object of the invention to provide such a system, wherein during fluoroscopy the X-ray tube filament current is controlled in accordance with a preselected value of filament current.

SUMMARY OF THE INVENTION A filament current control circuit varies the filament current of an X-ray tube in accordance with a control signal supplied to that circuit. The filament control signal is provided by a stabilization system comprising a comparator, and means for providing a reference signal and a comparison signal as two inputs to the comparator. The filament control signal, which is provided by the comparator, is proportional to the difference between the reference and comparison signals supplied to the comparator.

In a pre-exposure mode of operation, X-ray tube filament current feedback provides a comparison signal to the comparator which is proportional to the actual filament current. The reference signal provided to the comparator is a combination of two signals in that mode of operation. One of the signals is proportional to X-ray tube filament current required to provide a selected X-ray tube current. That signal is combined with another signal, which is proportional to required filament current to obtain the pre-selected tube current at a desired high voltage level.

In the exposure mode of operation, the reference signal provided to the comparator is proportional to a desired X-ray tube current, and the comparison signal provided to the comparator is a feedback signal proportional to the actual X-ray tube current. In this respect, the present system operates in a manner similar to that of previously known systems.

In a fluoroscopic mode of operation, the reference signal to the comparator is proportional to a desired filament current, and the comparison signal to the comparator is a feedback signal proportional to the actual X-ray tube filament current.

Both the reference and comparison signals provided to the comparator are in the form of current signals. The comparison signal proportional to actual X-ray tube filament current is obtained from current in a secondary winding of a transformer Whose primary winding is connected in the tube filament supply circuit. The comparison signal proportional to actual X-ray tube current may be obtained from a current measuring circuit such as that described in the reference application Ser. No. 708,963.

The reference signal proportional to required filament current to provide a selected current through the X-ray tube (in a pre-exposure mode) is obtained from one of a first plurality of resistors connected between a fixed voltage source and the input to the comparator. The signal proportional to required filament current for a desired level of high voltage to be supplied to the X-ray tube (in preexposure mode) is obtained from a metering circuit connected to the high voltage supply for the tube. The reference signal proportional to a desired current through the X-ray tube (in actual exposure mode) is obtained from one of a second plurality of resistors connected between the constant voltage supply and the input to the comparator. The signal proportional to desired filament current during fluoroscopy is obtained from a variable resistor also 3 connected between the constant voltage source and the input to the comparator.

Suitable switching is provided to insure that the proper reference and comparison signals are supplied to the comparator in the various modes of operation of the equipment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram, with some parts shown schematically, of one embodiment of the invention;

FIGS. 2a and 2b are graphs useful in understanding the invention; and

FIG. 3 is a detailed schematic diagram of the embodiment shown in block form in FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 illustrates a system embodying the invention for stabilizing current in an X-ray tube The X-ray tube 10 is preferably of a grid-controlled type having a control grid 10G, a filament 10F (cathode), and an anode 10A. With the gridcontrolled type of X-ray tube, high voltage may be supplied continuously between the anode 10A and the filament 10F, and the exposure controlled by a potential applied between the control grid 10G and the filament 10F. Thus, the kilovoltage (kv.) applied across the tube when the exposure is made is known and is available for control purposes before the exposure is made.

The high voltage for the X-ray tube 10 is provided and the exposure controlled by control circuitry 12. The present invention is well adapted to be used with a control system such as that described in application Ser. No. 651,835. The anode 10A and the filament 10F of the X-ray tube are respectively connected to positive and negative potentials through the control circuitry 12. The filament 10F is heated by means of a filament transformer 14 having a primary winding 14P connected between input terminals 16. A conventional 60-cycle, llO-volt source (not shown) may be connected to the terminals 16. A secondary winding 148 of the transformer 14 has one end connected through a variable resistor 18 to one side of the filament 10F. The other end of the secondary winding 145 is connected through a primary winding 20F of a transformer 20 to the other side of the filament 10F. The variable resistor 18 serves to limit the current flow through the filament to a predetermined value in the event of failure of the stabilization system to be described.

A center-tapped secondary winding 205 of the transformer 20 is connected into a. filament controller 22. The function of the filament controller 22 is to vary the current in the secondary winding 20S of the transformer 20, and hence vary the impedance reflected back into the primary winding 20F. This reflected impedance in turn controls the current flowing through the filament 10F of the X-ray tube 10. It is pointed out that the filament controller 22 is capable only of varying the impedance reflected into the primary winding P below a certain level. Thus, the current in the filament 10F can be varied only above a predetermined minimum level. That minimum level is generally determined by the maximum current through the X-ray tube that will be required, and by the maximum high voltage that will be applied across the X-ray tube in the course of an exposure. The filament controller 22 also provides an output signal on a lead 24 that is proportional to the actual filament current in the X-ray tube filament 10F.

The lead 24 is connected to one contact 26A of a twocontact, single-pole switch 26. The other contact 26B of the switch 26 is connected to a lead 28 to which the control circuitry 12 supplies a signal that is proportional to the actual current between the anode and cathode of the X-ray tube 10 when the tube is energized. The pole of the switch 26 is connected to one input of a comparator 30. Output of the comparator 30 is supplied on a lead 32 as input to the filament controller 22. Another input to the comparator 30 is provided from a pole of a single- 4 pole switch 34 having two contacts 34A, 34B. The switch 34 provides various reference signals to the comparator 30 to be compared with comparison signals supplied as a second input from the switch 26.

It will be appreciated that, although the switches 26, 34 and one to be described are shown as simple mechanical switches, they may well be electronic switches. Such switches are well-known in control circuitry, and in the present case would be part of the control logic for the entire X-ray apparatus. The control logic assures that the switches are in proper positions in the various modes of equipment operation to provide the proper reference and comparison signals to the comparator 30. The required positions of the various switches will become apparent as the description of an embodiment of the invention proceeds.

The contact 34A of the switch 34 is connected by means of a lead 36 to a kv. reference signal source 38, and by means of a lead 40 to an ma. reference signal source 42. The kv. reference source 38, which is essentially a function generator, receives a signal on a lead 44 from the control circuitry 12 and converts it into a suitable form to be used as input to the comparator 30. The signal received by the kv. reference signal source 38 from the control circuitry 12 is proportional to the high voltage that has been selected to be applied across the X-ray tube 10 during an ensuing X-ray exposure. This signal represents the X-ray tube filament current required for an exposure at that voltage level and at a certain predetermined minimum milliamperage level of X-ray tube current. Thus, that signal must be modified (increased) for different desired levels of X-ray tube current (ma.) above that minimum level.

The required modification of the kv. reference signal is provided by means of the ma. reference signal source 42. The ma. reference signal source 42 contains means for generating a plurality of signals proportional to required X-ray tube filament current necessary to provide various levels of X-ray tube current. In other words, the ma. reference signal selects a certain curve of kv. versus filament current, and the kv. reference signal selects the operating position on that curve.

The functions of the kv. reference signal source 38 and the ma. reference signal source 42 will be better understood by consideration of FIGS. 2a and 2b. It is well known that for presently available X-ray tubes a graph of tube current (ma.) versus filament current (FIL 1) provides a family of curves 44A-44E, such as is shown in FIG. 2a. The five curves 44A-44E, which number of curves is merely shown for purposes of illustration, correspond to different high voltages applied to the X-ray tube, the curve 44B representing 120 kv., the curve 44C representing 70 kv., the curve 44D representing 50 kv., and the curve 44E representing 40 kv. Of course, an infinite number of curves can be plotted, one for each applied voltage level. If now, voltage (kv.) is plotted against filament current (FIL I) for a given current (ma.) in the X-ray tube, such as is represented by a broken line 46 in FIG. 2a, one curve of a family of curves 48A, B, C, such as is shown in FIG. 2b is obtained. Each of the curves 48 represents a different X-ray tube current, With, for eX- ample, the curve 48A representing a tube current of ma., the curve 48B representing a current of 200 ma., and the curve 48C representing a current of 300 milliamps. Of course, an infinite number of curves can again be drawn with one curve representing each tube current value. In the present invention, six such curves are utilized, although only three are shown in FIG. 2b.

In effect, the ma. reference signal source 42 determines which of the curves 48 shown in FIG. 2b will be operated on, and the kv. reference signal source 38 determines the operating point on the selected curve. This is done in the kv. reference signal source 38 by approximating all of the curves 48 by a curve comprising two linear segments 50A, 50B as shown in FIG. 2b. If a closer correspondence to the actual curve shape is desired, more than two linear segments may be utilized. However, it has been found in practice that an approximation by two linear segments is satisfactory. A signal is provided from the kv. reference signal source 38 corresponding to operation at a predetermined maximum voltage level, for example, 150 kv. That signal then increases as the selected voltage level decreases, in order to increase the filament current in the X-ray tube and maintain the tube current constant.

Reference signals corresponding to a desired filament current are provided in the manner just described during the pre-exposure mode of equipment operation. They are supplied through the switch contacts 34A to the reference signal input of the comparator 30. A comparison signal input is supplied to the comparator 30 from the filament controller 22 through the switch contact 26A, and is proportional to the actual filament current of the X-ray tube. The output signal supplied from the comparator 30 to the filament controller 22 is such as to cause the controller to vary the impedance of the transformer primary 20P in the filament supply circuit to cause the reference current signal and the comparison current signal to be equalized.

During an actual exposure, either radiographic or fluoroscopic, the position of the switch 34 is changed to provide a circuit through the contact 34B. In a fluoroscopic mode of operation, the switch 26 stays in the position shown, so that a comparison signal proportional to actual mm filament current is provided to the comparator 30. The contact 34B of the switch 34 is connected to a pole of a single-pole switch 52 having two contacts 52A, 52B. The contact 52A of the switch 52 is connected to the output of a tube current reference signal source 54, which provides a signal proportional to a desired X-ray tube current. The switch contact 52B is connected to the output of a filament current reference signal source 56, which provides a signal proportional to a desired filament current.

As previously mentioned, in the fluoroscopic mode of operation of the equipment, the switch 26 is in the position shown. Thus, the comparison signal provided to the comparator 30 is a signal that is proportional to the actual X-ray tube filament current. The switch 52 is also in the position shown, so that the reference signal provided to the comparator is a signal that is proportional to a desired filament current for fluoroscopy.

During a radiographic exposure, the positions of both the switches 26 and 52 are changed. In that situation, the comparison signal supplied to the comparator is through the switch contact 26B and is proportional to X-ray tube current. Thus the comparator 30 provides an output signal to the filament controller 22 to adjust the current through the filament F of the X-ray tube 10 to vary the current through the X-ray tube until the comparison signal is equal to the reference signal. Although the number of tube current reference signals that may be supplied may be varied over a wide range, it has been convenient to selectively apply only one of six signals. In this manner, only a single selector switch need be used in cooperation with both of the reference signal sources 42 and 54.

FIG. 3 is a schematic diagram of the filament controller 22, the comparator 30, the kv. reference signal source 38, the ma. reference signal source 42, the tube current reference signal source 54, and the filament current reference signal source 56, all shown in block form in FIG. 1.

The filament controller 22, which is seen at the right side of FIG. 3, comprises three PNP transistors 100, 102, 104. The base of the transistor 100 is connected to the lead 32 to receive the output signal from the comparator 30. The collector of the transistor 100 is grounded, and its emitter is connected to a source of +D.C. voltage (not shown) through a load resistor 106. Current through the transistor 100, which is controlled by the comparator output signal on the lead 32, controls the potential applied to the bases of the transistors 102, 104. This, in turn, controls current in the secondary winding S of the transformer 20, whose primary winding 20P is connected in the filament current supply circuit.

The bases of the transistors 102, 104 are respectively connected through resistors 108, to the emitter of the transistor 100. The collectors of the transistors 102, 104 are respectively connected through diodes 112, 114 to opposite ends of the secondary winding 208 of the transformer 20. The emitters of transistors 102, 104 are con nected together and to the +D.C. source through a Zener diode 116. A potentiometer 118 is connected as a vari able resistor between the collector and the emitter of the transistor 102, and a potentiometer 120 is similarly connected across the transistor 104. The emitters of the transistors 102, 104 are also connected to a center tap on the transformer secondary winding 20S through a resistor 122 and a diode 124 connected in series. A potentiometer 126 is connected across the diode 124. The purposes of the resistor 122, the diode 124, and the potentiometer 126 will be later explained in detail in connection with deriving a signal proportional to the actual X-ray tube filament current.

60-cycle alternating current is induced in the secondary winding 208 by filament current flowing in the primary winding 20P of the transformer 20. During one-half cycle, current flows from the center tap of the secondary winding 20S through the diode 124, the resistor 122, and either the transistor 102 or the variable resistor 118. During the other half cycle, current must similarly flow through the other transistor 104 or the variable resistor 120. Thus, a pulsating 120-cycle voltage is developed across the resistor 122 that is proportional to current in the filament of the X-ray tube.

As the potential applied to the bases of the transistor 102, 104 varies, the emitter-collector resistances of the transistor vary, and thus the impedance in the secondary winding circuit of the transformer 20 is varied. As previously pointed out, this impedance is reflected into the primary winding 201 to vary the current supplied to the X-ray tube filament. The purpose of the variable resistors 118, 120 is to provide a predetermined maximum impedance in the transformer secondary circuit when the transistors 102, 104 are completely non-conductive. As the transistors 102, 104 conduct, they effectively decrease the resistances of the resistors 118, 120, this resulting in decreasing the impedance of the transformer secondary winding circuit. This, in turn, results in increasing the X-ray tube filament current.

A voltage signal proportional to X-ray tube filament current is derived from the voltage drop across the resistor 122. To this is added a portion of the small signal appearing across the diode 124. The emitters of the transistors 102, 104 are connected to ground through a parallel combination of a resistor 128 and a capacitor 130, and are connected directly to the base of a PNP transistor 132. The emitter of the transistor 132 is connected through a resistor 134 to an adjustable pick-off arm of the potentiometer 126. The collector of the transistor 132 is connected to ground through a fixed resistor 136 and two variable resistors 138, 140, all three resistors being connected in series. A diode 142 and a capacitor 144 are connected in parallel across the resistor chain from the collector of the transistor 132 to ground.

It is seen that the transistor 132 is connected in a common-base configuration, so that its collector current is approximately equal to its emitter current. Thus, the current flowing in its collector circuit is approximately proportional to the actual filament current in the X-ray tube. The purpose of the potentiometer 126 is to provide a slight additional voltage to compensate for the base-emitter drop in the transistor 132. The collector of the transistor 132 is connected through a resistor 146 and a diode 148 in series to the lead 24, which it will be recalled is one of the input leads to the comparison input of the comparator 30.

It is pointed out that a juncture between the variable resistors 138, 140 is connected through a resistor 150 toa D.C. voltage source (not shown). The variable resistor 138 permits the volts per filament-ampere relation to be somewhat adjustable, which is required because of nominal tolerance build-ups in the conversion system. The adjustment; of the variable resistor 140 controls the amount of negative reference voltage supplied to the low or reference end of the resistor 138. This negative voltage forms an artificial base level for the derived voltage, thus permitting a finite X-ray tube filament current to flow with zero volts produced at the collector of the transistor 132. This arrangement permits a wider volts per filamentampere swing in the positive or above-zero useful voltage range. The capacitor 144 serves as an integrator, which is necessary since the current appearing at the collector of the transistor 132 is a 120-cycle pulsating DC. current.

An NPN transistor 152 and the diode 148 previously mentioned constitute a gate that permits the comparison signal current to flow from the collector of the transistor 132 through the lead 24 to the comparison input of the comparator 30. The collector of the transistor 152 is connected to a juncture between the resistor 146 and the anode of the diode 148. The emitter of the transistor 152 is grounded. The base of that transistor is connected to a source of +D.C. voltage (not shown) through a resistor 153, and to ground through a switch 154. When the switch 154 is closed, the base and emitter of the transistor 152 are both at ground potential, and the transistor is non-conductive. Thus current flows through the diode 148 and into the comparator comparison input. When the switch 154 is open, the base of the transistor 152 is positive with respect to its emitter, so that the collector-emitter circuit of that transistor effectively short circuits to ground the comparison current signal from the transistor 132, and prevents its being applied to the comparison input of the comparator The comparator 30 is shown in the center of FIG. 3. It comprises four NPN transistors 154, 156, 158, 160 and a PNP transistor 162. The transistors 154, 160 serve as input transistors to increase the input impedance of the comparator, and the transistor 162 serves as an output driver. The transistors 156, 158 comprise the actual comparator.

The base of the input transistor 154 is connected to ground through a resistor 164. Comparison input signals are provided to the base of the transistor 154 either through the lead 24 from the transistor 132, or through the lead 28 connected to the base of the transistor 154 through a switch 166. The collector of the transistor 154 is connected directly to the +D.C. source, and its emitter is connected to the D.C. source through a resistor 168. The emitter is also connected directly to the base of the comparator transistor 156.

The base of the input transistor 160 is connected to ground through a resistor 170, and is also connected to receive reference signals through a lead 172. The collector of the transistor 160 is connected directly to the +D.C. source. Its emitter is connected to the -D.C. source through a resistor 174 and is connected directly to the base of the comparison transistor 158.

The collector of the comparison transistor 156 is connected directly to the +D.C. source. The collector of the other comparison transistor 158 is connected to the +D.C. source through a load resistor 176, and is connected directly to the base of the driver transistor 162. The emitters of the transistors 156, 158 are connected together and to the D.C. source through a resistor 178.

The collector of the driver transistor 162 is connected directly to ground. Its emitter is connected to the -+D.C. source through a load resistor 180, and is connected directly to the base of the transistor 100 in the filament controller 22 by means of the lead 32. Thus, the degree of conduction of the driver transistor 162 determines the potential applied to the base of the transistor 100 in the filament controller, which, in turn, controls the potentials 8 applied to the bases of the shunt transistors 102, 104. The degree of conduction of the transistors 102, 104, as previously mentioned, controls current in the secondary winding 20S of the transformer 20, and hence controls the impedance and current flow in the filament primary current supply.

When the comparator 30 is balanced, i.e., when the potentials at the bases of the transistors 154, 160 are equal, current flows through both transistors 156, 158. Thus, due to the voltage drop across the resistor 176, the base of the driver transistor 162 is negative with respect to the emitter of that transistor. This causes the transistor 162 to conduct and, in turn, causes the transistor to conduct. This decreases the potential applied to the bases of the shunt transistors 102, 104 and causes them to conduct. Thus, a particular impedance is reflected into the primary circuit of the filament current supply. If the comparison signal applied to the base of the transistor 154 becomes less than the reference signal applied to the base of the transistor 160, due to a change in either one of the two signals, the transistors 154, 156 will conduct less heavily, while the transistors 158, will conduct more heavily. This causes the driver transistor 162 to conduct more heavily and hence increases conduction of the transistor 100 in the filament controller. This, in turn, causes the shunt transistors 102, 104 to conduct more heavily and decreases the impedance in the secondary Winding circuit of the transformer 20. This decrease in impedance is reflected into the primary winding circuit and hence to the filament current circuit for the X-ray tube to increase the filament current. The converse, of course, is also true. If the comparison signal exceeds the reference signal, conduction of the transistors 158, 160, 162, 100, 102, 104 will be decreased. This results in increasing the impedance in the filament current supply circuit and decreasing the filament current.

As previously mentioned, the comparison signal may be supplied to the base of the transistor 154 either through the lead 24' or through the switch 166 in the lead 28. In the first case, the switch 154 would be closed, so that the transistor 152 is non-conductive. Thus, the signal supplied on the lead 24 is one that is proportional to the actual filament current in the X-ray tube. If, in another phase of operation, the switch 154 is open, that filament current comparison signal will be shorted out through the transistor 152. Simultaneously, the control logic circuitry of the apparatus causes the switch 166 to close, so that a signal proportional to actual X-ray tube current appears across the resistor 164 and hence at the base of the input transistor 154. How the signal proportional to actual X-ray tube current is obtained is of no particular consequence so far as the invention is concerned. However, it is pointed out that it may well beobtained from the current measuring circuit described in the referenced application Ser. No. 708,963. In actual practice, this arrangement has been used, and is preferred.

As previously mentioned, a reference signal may be supplied from the kv. reference signal source 38 and the ma. reference source 42, the two signals being supplied slmultaneously and combined. Those signals are supplied to the comparator 30 through a PNP switching and comblnlng transistor 182. The transistor 182 is connected in a common-base configuration, with its emitter connected to the leads 36 and 40 from the two reference signal sources 38, 42, and its collector connected to the lead 172 and hence to the base of the transistor 160 in the comparator 30. The base of the transistor 182 is connected to ground through a resistor 184 and a switch 186 connected in series. The base is also connected to the +D.C. source through a Zener diode 188 and a resistor 190 connected in parallel. Thus, if the switch 186 is closed, the transistor 182 is conductive and its collector current is approximately equal to its emitter current. If the switch 186 is open, the transistor 182 is non-conductive and the reference signals appearing on the leads 36, 40- are not 9 connected through the transistor to the lead 172. The transistor 182 thus acts as a switch that is open or closed, depending on the state of the switch 186 in its base circuit.

Reference signals from the tube current reference signal source 54 and from the filament current reference signal source 56 are connected directly to the lead 172 through switches 192,194, respectively. The signal sources 54, 56 will be later described in detail.

The kv. reference signal source 38, which serves as a function generator, is shown at the left of FIG. 3. Its input comprises a signal transformer 200 having a primary winding 200P and a secondary winding 2008. The primary winding 200P is connected at input terminals 202 to receive a selected radiographic high voltage. This is normally supplied from an autotransformer. The transformer secondary winding 200$ has one end grounded, and its other end connected through a variable resistor 204 to the emitter of an NPN transistor 206. The base of the transistor 206 is connected to ground through a diode 208, and to a +D.C. voltage source (not shown) through a resistor 2 10. Thus, the base of the transistor 206 is maintained at a constant positive potential due to the small voltage drop across the diode 208. A diode 212 is connected between the emitter of the transistor 206 and ground, and serves to clamp any positive excursions of the input voltage at the emitter of the transistor 206. This insures that the base-emitter junction of the transistor will not be broken down in the reverse direction.

The transistor 206 is connected in a common-base configuration. Its collector is connected through a series arrangement comprising a potentiometer 214, a fixed resistor 216, and a DC milliamrneter 218 (which acts as a kv. meter) to the +D.C. supply. A Zener diode 220 has its anode connected to a juncture of the potentiometer 214 and the resistor 216, and its cathode connected to a juncture between the resistor 216 and the meter 218. Thus, the Zener diode 220 is connected to act as an open switch across the resistor 216 until the potential drop across that resistor has reached a predetermined level. At that level, the diode 220 breaks down and acts as a resistor having a relatively low resistance value. A capacitor 221 is connected between the collector of the transistor 206 and the +D.C. supply to integrate the pulsating D.C. appearing at the collector of the transistor.

An adjustable arm of the potentiometer 214 is connected to the base of an NPN transistor 222 through a resistor 224. The transistor 222 is connected as an emitterfollower, with its collector connected to a +D.C. voltage supply (not shown), and its emitter connected through a diode 226 and a variable resistor 228 in series to the lead 36. The base of the transistor 222 is also connected to ground through an integrating capacitor 230.

In operation, current through the kv. meter 218 also fiows through the potentiometer 214 and the parallel combination of the resistor 216 and the Zener diode 220. When the 'kilovoltage is low, the diode 220 does not break down and the voltage change at the collector of the transistor 206 corresponding to a given kilovoltage change is rather rapid. At higher kilovoltages, however, the diode.220 breaks down, and the change in potential at the collector of the transistor 2.06 for a given change of kilovoltage becomes much less. A portion of that voltage is picked off by the adjustable arm of the potentiometer 214 and applied to the base of the emitter-follower transistor 222. This is illustrated by the straight line segments 50A, 50B shown in FIG. 2b. This voltage signal is converted to a current by the transistor 222 and applied through the lead 36 to the emitter of the switching transistor 182 along with the ma. reference signal applied through the line 40.

In the kv. reference signal source 38, the adjustable resistor 204 serves as a meter calibration adjustment for the meter 218. The potentiometer 214 serves to adjust the slope of the high voltage portion of the response curve, and the variable resistor 228 serves to adjust the slope of the low voltage portion of the response curve. In other words, adjustment of the potentiometer 214 affects the slope of the curve segment 50A, and adjustment of the variable resistor 228 affects the slope of the curve segment 50B (FIG. 2b).

As previously stated, the reference signal supplied on the line 40 to the emitter of the switching transistor 182 from the ma. reference signal source 42 is a static equivalent of the required filament current to produce a selected X-ray tube current at kv. applied across the tube. This is referred to as a base level. The ma. reference signal source comprises a plurality of adjustable resistors 240A-240F which are respectively connected through diodes 242A-242F to contacts 244A-244F of a six-position selector switch 244 having a pole 244P. The pole 244P of the switch is connected to a +D.C. voltage source (not shown). Each position of the switch 244 corresponds to a different selected value of X-ray tube current with 15 0 kv. applied across the tube. For example, when a circuit is completed through the selector switch contact 244A, the variable resistor 240A may be adjusted to provide a reference current on the lead 40 that corresponds to a tube current of 300 milliamps. The other variable resistors 240B-240F may be adjusted to provide X-ray tube filament currents corresponding to various desired X-ray tube currents, all being adjusted for their desired currents at 150 kv. or some other fixed, predetermined maximum kv. level applied across the X-ray tube. Thus, it is apparent that currents through the leads 36 and 40 will simultaneously flow through the switching transistor 182 and through the lead 172 and cause a reference signal .to appear across the resistor 170 at the base of the input transistor in the comparator 30. As the high voltage across the X-ray tube is changed, the signal applied to the base of the transistor 160 will follow a two-segment approximation of one of the curves 48 shown in FIG. 2b. If .a different tube current level is selected by means of the selector switch 244, a different one of the curves 48 will be chosen, which will then again be followed as the high voltage applied to the tube is changed.

If it is desired to change the reference current signal from the kv. reference signal source 38 and the ma. reference signal source 42, the switch 186 in the base circuit of the switching transistor 182 is opened. This causes the base of that transistor to go a high potential level and effectively opens the switch connecting the lead 172 to the leads 36, 40. When that is done, a reference current signal may be supplied to the lead 172 through the switch 192 from the tube current reference source 54, or through the switch 194 from the filament current reference source 56.

The tube current reference source 54 comprises a plurality of variable resistors 246A-246F, all connected together at one end to one contact of the switch 192. The other ends of the resistors 246A-246F are respectively connected through diodes 248A-248F to the contacts 244A-244F of the selector switch 244. The variable resistors 246A-246F are adjusted to provide currents through the switch 192 to the lead 172 that are respectively proportional to desired X-ray tube currents determined by the various settings of the selector switch 244. In other words, if a circuit is completed through the selector switch contact 244A, for example, current will be provided through the lead 40 that is proportional to a desired fila ment current to provide 300 milliamperes of tube current at 150 kv. applied across the X-ray tube. Current will also be supplied to one contact of the switch 192., which is pro portional to a tube current of 300 rnilliamperes, regardless of the voltage applied across the tube. If the switch 186 is closed and the switch 192 is open, the first current will be supplied as part of the reference signal input to the comparator 30. If the conditions are reversed and the switch 186 is open and the switch 192 is closed, the second current will be supplied as reference signal input to the comparator.

The filament current reference signal source 56 comprises a variable resistor 250 connected between a +D.C. voltage source (not shown) and one contact of the switch 194. If the switch 192 is open and the switch 194 is closed (and it is inherent in the equipment control logic that the switches 192, 194 cannot be in the same condition simultaneously), a current determined by the adjustment of the resistor 250 will be supplied through the lead 172 as a reference signal input to the comparator 30.

In summarizing, it is noted that the embodiment of the X-ray tube current stabilization system illustrated and described has three modes of operation. One of these is a pre-exposure mode for radiography. In that mode, a reference signal proportional to desired filament current for a particular tube current and kilovoltage applied to the tube is compared with the actual filament current. In order to accomplish this, the equipment primary logic causes the switch 154 to be closed so that the transistor 152 is non-conductive and a comparison signal proportional to the actual filament current is provided through the lead 24 to the base of the transistor 154 in the comparator. At this time the switch 166 is open. In this preexposure mode, reference signals are provided from the kv. reference signal source 38 on the lead 36 and from the ma. reference signal source 42 on the lead 40 to the emitter of the switching transistor 182. The switch 186 in the base circuit of that transistor is closed so the current flows through the transistor and provides the reference signal at the base of the transistor 160 at the comparison signal input of the comparator 30.

A second mode of operation is the actual exposure mode, which occurs while a radiograp-h is being made. In this mode, the actual X-ray tube current is compared with a reference X-ray tube current and the filament current adjusted to make the two quantities equal. The primary logic causes the switch 154 to open so that the transistor 152 effectively connects the actual filament current signal to ground. The switch 166 is closed to apply a signal proportional to actual tube current to the comparison input to the comparator 30. The switch 186 is opened to cause the transistor 182 to become nonconductive and remove from the comparator the reference signals appearing on the leads 36 and 40. The switch 192 is closed to provide on the lead 172 and at the reference signal input of the comparator 30 a signal proportional to desired tube current. The output of the comparator 30 through the driver transistor 162 causes the filament controller 22 to adjust the filament current to provide the required tube current.

A third mode of operation is the fluoroscopic mode. In this mode, the actual filament current is compared with a desired filament current and the former adjusted until the two are equal. The switch 154 is again closed to cause the transistor 152 to become non-conductive and a signal is app-lied to the comparison signal input of the comparator 30 that is proportional to the actual filament current. The switch 166 is open, and the switch 186 is open. The switch 192 is opened to remove the signal from the tube current reference source 54. The switch 194 is closed to apply to the lead 172 the signal from the filament current reference source 56. Thus, two signals respectively proportional to a desired filament current and an actual filament current are compared in the comparator 30. Its output causes the filament controller 22 to adjust the impedance in the filament supply circuit so that the actual filament current is equal to the desired filament current.

Although only one embodiment of the invention has been shown and described, it is apparent that many changes and modifications may be made by one skilled in the art without departing from the true spirit and scope of the invention.

I claim:

1. In an X-ray apparatus having an Xray tube and a filament current control circuit for varying filament current of said X-ray tube in accordance with a filament control signal sup-plied thereto, a tube current stabilization system for providing said control signal, comprising:

(a) comparator means having a reference input for receiving a filament current reference signal and a comparison input for receiving a comparison signal and providing said control signal which is proportional to a difference between said filament current reference and comparison signals;

(b) a milliampere reference signal source for providing a milliampere reference signal proportional to filament current required to provide a preselected tube current at a predetermined high voltage applied across said tube;

(c) means for modifying said milliampere reference signal in accordance with additional filament current required to provide said preselected tube current at a preselected high voltage less than said predetermined high voltage to provide said filament current reference signal; and

(d) current responsive means responsive to actual filament current for providing said comparison signal.

2. In an X-ray apparatus having an X-ray tube and a filament current control circuit for varying filament current of said X-ray tube in accordance with a filament control signal supplied thereto, a tube current stabilization system for providing said control signal, comprising:

(a) comparator means having a reference input for receiving a filament current reference signal and a comparison input for receiving a comparison signal and providing said control signal, which is proportional to a difference between said filament current reference and comparison signals;

(b) a milliampere reference signal source for providing a milliampere reference signal proportional to filament current required to provide a preselected tube current at a predetermined high voltage applied across said tube;

(c) a high voltage reference signal source for providing a high voltage reference signal related to additional filament current required to provide said preselected tube current at a preselected high voltage less than said predetermined high voltage;

(d) means for combining said millampere reference signal and said high voltage reference signal to provide said filament current reference signal; and

(e) current responsive means responsive to actual filament current for providing said comparison signal.

3. The system of claim 2, wherein said milliampere reference signal, said high voltage reference signal and said comparison signal are current signals.

4. The system of claim 3, wherein said means for combining comprises a transistor connected in common-base configuration.

5. The system of claim 2, wherein said high voltage reference signal source comprises a function generator.

6. The system of claim 5, wherein said function generator provides said high voltage reference signal in accordance with an approximation of a high voltage-filament current curve for a predetermined minimum tube current.

7. The system of claim 6, wherein said approximation .of said curve comprises a plurality of substantially straight lines.

8. In an X-ray apparatus having an X-ray tube and a filament current control circuit for varying filament current of said X-ray tube in accordance with a filament control signal supplied thereto, a tube current stabilization system for providing said control signal, comprising:

(a) comparator means having a reference input for receiving a filament current reference signal and a comparison input for receiving a comparison signal and providing said control signal which is proportional to a difference between said filament current reference and comparison signals;

(b) a milliampere reference signal source for providing a milliampere reference signal proportional to filament current required to provide a preselected tube current at a predetermined high voltage applied across said tube;

(c) means for modifying said milliampere reference signal in accordance with additional filament current required to provide said preselected tube current at a preselected high voltage less than said predetermined high voltage to provide a first filament current reference signal;

(d) first current responsive means responsive to actual filament current for providing a first comparison signal;

(e) a tube current reference signal source for providing a second filament current reference signal proportional to said preselected tube current;

(f) second current responsive means responsive to actual tube current for providing a second comparison signal; and

(g) switching means for selectively and simultaneously supplying said first filament current reference signal and said first comparison signal to said comparator means before a radiographic exposure, and for selectively and simultaneously supplying said second filament current reference signal and said second comparison signal to said comparator means during said radiographic exposure.

9. In an X-ray apparatus having an X-ray tube and a filament current control circuit for varying filament current of said X-ray tube in accordance with a filament control signal supplied thereto, a tube current stabilization system for providing said control signal, comprising:

(a) comparator means having a reference input for receiving a filament current reference signal and a comparison input for receiving a comparison signal and providing said control signal which is proportional to a difference between said filament current reference and comparison signals;

(b) a milliampere reference signal source for providing a milliampere reference signal proportional to filament current required to provide a preselected tube current at a predetermined high voltage applied across said tube;

(c) a high voltage reference signal source for providing a high voltage reference signal proportional to additional filament current required to provide said preselected tube current at a preselected high voltage less than said predetermined high voltage;

(d) means for combining said milliampere reference signal and said high voltage reference signal to provide a first filament current reference signal;

(e) first current responsive means responsive to actual filament current for providing a first comparison signal;

(f) a tube current reference signal source for providing a second filament current reference signal proportional to said preselected tube current;

(g) second current responsive means responsive to actual tube current for providing a second comparison signal; and

(h) switching means for selectively and simultaneously supplying said first filament current reference signal and said first comparison signal to said comparator means before a radiographic exposure, and for selectively and simultaneously supplying said second filament current reference signal and said second comparison signal to said comparator means during said radiographic exposure.

10. The system of claim 9, wherein said first and second filament current reference signals and said first and second comparison signals are current signals.

11. The system of claim 10, wherein said means for combining comprises a transistor connected in commonbase configuration.

12. The system of claim 9, wherein said high voltage reference signal source comprises a function generator.

13. The system of claim 12, wherein said function generator provides said high voltage reference signal in accordance with an approximation of a high voltage-filament current curve for a predetermined minimum tube current.

14. The system of claim 13, wherein said approximation of said curve comprises a plurality of substantially straight lines.

15. In an X-ray apparatus having an X-ray tube and a filament current control circuit for varying filament current of said X-ray tube in accordance wtih a filament control signal supplied thereto, a tube current stabilization system for providing said control signal, comprising:

(a) comparator means having a reference input for receiving a filament current reference signal and a comparison input for receiving a comparison signal and providing said control signal which is proportional to a difference between said filament current reference and comparison signals;

(b) a milliampere reference signal source for providing a milliampere reference signal proportional to filament current required to provide a preselected tube current at a predetermined high voltage applied across said tube;

(c) means for modifying said milliampere reference signal in accordance with additional filament current required to provide said preselected tube current at a preselected high voltage less than said predetermined high voltage to provide a first filament current reference signal;

((1) first current responsive means responsive to actual filament current for providing a first comparison signal;

(e) a tube current reference signal source for providing a second filament current reference signal proportional to said preselected tube current;

(f) second current responsive means responsive to actual tube current for providing a second comparison signal;

(g) a filament current reference signal source for providing a third filament current reference signal proportional to a desired filament current; and

(h) switching means for selectively and simultaneously supplying said first filament current reference signal and said first comparison signal to said comparator means before a radiographic exposure, for selectively and simultaneously supplying said second filament current reference signal and said second comparison signal to said comparator means during said radiographic exposure, and for selectively and simultaneously supplying said third filament current reference signal and said first comparison signal to said comparator means during a fluoroscopic exposure.

16. In an X-ray apparatus having an X-ray tube and a filament current control circuit for varying filament current of said X-ray tube in accordance with a filament control signal supplied thereto, a tube current stabilization system for providing said control signal, comprising:

(a) comparator means having a reference input for receiving a filament current reference signal and a comparison input for receiving a comparison signal and providing said control signal which is proportional to a difference between said filament current reference and comparison signals;

(b) a milliampere reference signal source for providing a milliampere reference signal proportional to filament current required to provide a preselected tube current at a predetermined high voltage applied across said tube;

(c a high voltage reference signal source for providing a high voltage reference signal proportional to additional filament current required to provide said pre- 15 selected tube current at a preselected high voltage less than said predetermined high voltage;

(d) means for combining said milliampere reference signal and said high voltage reference signal to provide a first filament current reference signal;

(e) first current responsive means responsive to actual filament current for providing a first comparison signal;

(f) a tube current reference signal source for providing a second filament current reference signal proportional to said preselected tube current;

(g) second current responsive means responsive to actual tube current for roviding a second comparison signal;

(h) a filament current reference signal source for providing a third filament current reference signal proportional to a desired filament current; and

(i) switching means for selectively and simultaneously supplying said first filament current reference signal and said first comparison signal to said comparator means before a radiographic exposure, for selectively and simultaneously supplying said second filament current reference signal and said second comparison signal to said comparator means during said radiographic exposure, and for selectively and simultaneously supplying said third filament current reference signal and said first comparison signal to said comparator means during a fluoroscopic exposure.

17. The system of claim 16, wherein said first, second and third filament current reference signals and said first and second comparison signals are current signals.

18. The system of claim 17, wherein said means for combining comprises a transistor connected in commonbase configuration.

19. The system of claim 16, wherein said high voltage reference signal source comprises a function generator.

20. The system of claim 19, wherein said function generator provides said high voltage reference signal in accordance with an approximation of a high voltage-filament current curve for a predetermined minimum tube current.

21. The system of claim 20, wherein said approximation of said curve comprises a plurality of substantially straight lines.

22. The system of claim 4, wherein said milliampere reference signal source comprises a plurality of current sources selectively connectable in an input circuit of said transistor.

23. The system of claim 4, wherein said high voltage reference signal source comprises a function generator having a current output connected in an input circuit of said transistor.

24. The system of claim 4, wherein said milliampere reference signal source comprises a plurality of current sources selectively connectable in an input circuit of said transistor, and said high voltage reference signal source comprises a function generator having a current output connected in said input circuit of said transistor.

25. The system of claim 11, wherein said milliampere reference signal source comprises a plurality of current sources selectively connectable in an input circuit of said transistor.

26. The system of claim 11, wherein said high voltage reference signal source comprises a function generator having a current output connected in an input circuit of said transistor.

27. The system of claim 11, wherein said milliampere reference signal source comprises a plurality of current sources selectively connectable in an input circuit of said transistor, and said high voltage reference signal source comprises a function generator having a current output connected in said input circuit of said transistor.

28. The system of claim 18, wherein said milliampere reference signal source comprises a plurality of current sources selectively connectable in an input circuit of said transistor.

M. The system of claim 18, wherein said high voltage reference signal source comprises a function generator having a current output connected in an input circuit of said transistor.

30. The system of claim 18, wherein said milliampere reference signal source comprises a plurality of current sources selectively connectable in an input circuit of said transistor, and said high voltage reference signal source comprises a function generator having a current output connected in said input circuit of said transistor.

References Cited UNITED STATES PATENTS 2,810,838 11/1957 Clapp et al 250103 RALPH G. NILSON, Primary Examiner C. E. CHURCH, Assistant Examiner 

